The dirty plumes travel around 10,000 kilometers: Transatlantic air currents often transport large quantities of soot particles from bushfires in Africa to the central Amazon rainforest, a study shows. The contribution from afar can even significantly exceed the load from the regional fires. The scientists say that the information can now be used to better assess atmospheric currents and the role of soot particles in the climate system.
In forests, the air is usually particularly fresh and clean – one might think this is particularly true of the vast Amazon rainforest. But fine dust is also in the air there. However, the concentration of soot particles fluctuates between low and very high. Especially in the rather dry time of the year, there are considerable loads in some areas. Because then numerous man-made deforestation fires will burn in the Amazon rainforest. As a result, the air quality in the central Amazon is sometimes hardly better than in European metropolitan areas.
On the trail of soot
But the research team led by Bruna Holanda from the Max Planck Institute for Chemistry in Mainz now wanted to know more precisely where the soot particles in the air in the Amazon region come from. Because contributions from more distant sources also seemed possible. In order to be able to assign the soot over the Amazon to different origins, the scientists analyzed particles in the air at the Amazon Tall Tower Observatory (ATTO) over a period of two years. This research center is located in a comparatively untouched region in the central Amazon region and includes, among other things, a 325 meter high measuring tower.
As the researchers report, they encountered two predominant types of soot during their analyses: one part was characterized by comparatively small particles with a high content of organic material. They come from the regional fires, the researchers explain. The characteristics of these soot particles can be traced back to comparatively moist fuels and smoldering fires. However, the test results of the second fraction caused a surprise: The relatively large and heavily burned particles come from Africa and were created there during bush fires, slash-and-burn and the burning of biomass, according to comparisons.
Amazingly great contribution
As the scientists found out, at some times even the majority of the pollution does not come from South America itself, but traveled around 10,000 kilometers across the Atlantic with air masses from Africa. “This smoke from Africa can be found in large proportions over the rainforest almost all year round – we didn’t expect that,” says Holanda. “We had estimated the proportion from Africa at five, maybe 15 percent. In fact, at times it was 60 percent.” Using meteorological data such as the main wind field and satellite images, the researchers then determined the more precise sources and paths of the plumes of smoke. Among other things, it turned out that twice a year a particularly large amount of smoke flows from Africa to the Amazon.
According to this, so much soot is transported during the rainy season from January to March that during this period an average of 60 percent of the soot particles over the Amazon come from the African fires. At this time there is hardly any smoke from local fire clearance in the air. But the smoke from Africa sometimes makes the air as dirty as it is during the dry season, the researchers say. The results show that the second strong transatlantic plume of smoke also significantly increases the load in the dry season: from August to November, the regional fires are responsible for around two thirds of the soot load. Soot from Africa contributes a third and thus increases the already high pollution, say the scientists.
But what effects can this entry of soot particles have? Various effects are possible, the researchers emphasize: Particles in the air influence sunlight and can thus influence the earth’s radiation or energy balance and thus the climate. Dust and soot particles also serve as condensation nuclei in the formation of droplets in clouds. They thus influence precipitation and the water balance. “Our results can therefore now help to improve climate and Earth system models, which so far have only inadequately reflected the African smoke component,” says senior author Christopher Pöhlker from the Max Planck Institute for Chemistry. “The deforestation rates and the number of fires and the resulting soot in recent years are unprecedented and can have serious consequences for regional and global climate change,” the scientist concludes.
Source: Max Planck Institute for Chemistry, Commun Earth Environ, doi: 10.1038/s43247-023-00795-5